Final: What makes the weather work in the tropics?

This discussion topic submitted by Willie Reed (
reedwa@miamioh.edu) at 10:02 am on 6/7/01. Additions were last made on Wednesday, May 7, 2014.
What makes the weather work in the tropics?

What makes the weather so extreme in the tropics? There are many factors involved in answering this question. Among a few are Trade Winds, Inter-Tropical Convergence Zones (ITCZ), and the circulation of the atmosphere due to the Coriolis effect. The amount of rainfall, El Nino, and tropical storms also play a major role in creating extreme weather conditions in the tropics. Extreme weather in the tropics is most prominent during hurricane season. Many of the worst hurricanes, such as Hurricane Andrew and Hurricane Floyd, make world headlines because of the catastrophic damage that they cause in the tropics. It is important to study and learn about tropical weather patterns. Many humans and other types of life forms live in the tropical regions of the Earth and they are all greatly affected by the weather patterns in these regions. The more everyone learns about tropical weather patterns, the safer it will be to inhabit the tropical regions of Earth. When looking at atmospheric circulation we must first get an overall view of it on a global level. The atmosphere of low latitudes receives more solar radiation than it re-radiates back to space, while latitudes greater than that in the region of 38 degrees re-radiates more than they receive. With this right amount of energy imbalance it drives the circulation of the atmosphere, which allows it to achieve the right amount of latitudinal energy transfer and maintain the atmospheric temperatures at a certain state. With the earth always rotating a stream of equatorial air at high altitudes flow toward the poles with a westerly velocity. As the air continues in this direction it begins to slow down as it blows over earth and the air pattern takes on an apparent westerly flow. This change in airflow is know as the Coriolis effect and seems to become more distinct as latitude increases. This also happens in the reverse direction, as well, and has the opposite effect on the Coriolis effect. The results of the Coriolis effect are a deflection of air movement to the right in the northern hemisphere, and to the left in the southern hemisphere. With this comes high velocity jet streams that carry major discontinuities between cold and warm air. By having these jet streams the cold and warm air masses allows the transfer of latitudinal energy that is required to maintain a thermal equilibrium in our atmosphere. Without these jet streams our atmosphere would get either very hot or very cold (Kellman, Tackaberry 27-28). The Trade Winds playing off of the Coriolis effect take on an easterly flow at the surface while the Coriolis effect takes on a westward flow at higher altitudes. In the northern hemisphere the trade winds move in a northeasterly direction, while in the southern hemisphere they move in a southeasterly direction. All of this takes place in an area called the Inter-Tropical Convergence Zone (ITCZ). Seasonal variation in the tropical circulation patterns plays a major role in the positioning of the ITCZ and subtropical high-pressure zones. These shifts are done in seasonal cycles and in response to seasonal changes in the solar angles and radiation heating. The largest amount of latitudinal shifts takes place over land, while over water the shifts are smaller, because the pattern of the sea surface temperature that controls the ITCZ is very stable. Because of this only very low-latitude sites in the tropics are affected by the ITCZ directly. Another major factor associated with the trade winds and the ITCZ, is Trade Wind Inversion. This is a vertical thermal irregularity that is embedded within the trade winds. The main effect of this is to repress spontaneous vertical movement of air, which depends upon a progressive decrease in the temperature with height. The inversion shows a steady seasonal pattern of variation, with the most extreme development happening in the hemispheric winters, and the weakest development during the summers. Due to its effect on convection, the Trade Wind Inversion has a great affect on the seasonal patterns of rainfall throughout most of the tropics (Kellman, Tackaberry 29-31). Rain we know comes from the condensation of water in the clouds, which in turn comes down as raindrops. In the tropics their weather seems to be on a small and local scale and there is a tendency for the rainstorms to be short in duration, but in high intensity. It is estimated that “40 percent of all tropical rainstorms exceed 25 mm.p-1 at a rate considered to be the threshold intensity at which rainfall becomes erosive” (Hudson 34). Most tropical areas experience some seasonality in rainfall, representing periods when convective activity is either suppressed or enhanced, due to much of the variation in rainfall totals and is recognized by the lengths of the two seasons. The wet and dry season include extended periods of several months where daily rainfall occurs with high and low frequencies. During these wet months the rainfall is affected by the trade winds and the intensification of the trade wind inversion, the ITCZ, which brings increased rainfall on the area, and the monsoons. Not only do the tropics experience lots of rain, but they also experience times when some areas get no rain at all which may lead to droughts. Most of these areas are northeastern Brazil, northwest India, and much of Africa. Yet most of the other places in the tropics get plenty of rain. Much of the rainfall in the tropics can be interpreted on a large scale, not forgetting that many of the local, smaller areas have their own small weather patterns that should not be overlooked (Kellman, Tackaberry 34, 39-43). “El Nino was originally recognized by fisherman off the coast of South America as the appearance of unusually warm water in the Pacific ocean, occurring near the beginning of the year.” El Nino means The Little Boy or Christ child in Spanish. This name was used for the tendency of the phenomenon to arrive around Christmas (Recognizing El Nino). Some of the more major El Ninos occurred in 1925-1926, 1939,1941, 1957-1958, 1972-1973, and 1982-1983. Over the past two decades, evidence suggests that much of the variabilities in rainfall throughout the tropics are somehow linked to ESNO or El Nino Southern Oscillation. ESNO occurs on an average of every four years; however, these cycles vary between two and ten years, and since 1990 there has been persistent warming. The most obvious biological consequence of an ESNO is the suppression of cold upwelling in the eastern Pacific, leading to a collapse of marine productivity. The absence of cold water and its nutrients leads to enormous death of fish, and the bird life which feeds on them, causing an economic disaster for the fishing industries of Ecuador, Peru, and northern Chile (Barry, Chorley 267-268). Other effects and places that an ESNO has produced damage include forest fires in Borneo that occurred during 1982-1983, and fires in the Galapagos Islands in 1984 that were brought on by the drying of biomass that had built up during the preceding wet event. Although most of the direct climate effects have been felt in the Pacific basin, there is an abundant amount of evidence pointing to links between ESNO and weather conditions elsewhere in the tropics and beyond the tropics (Kellman, Tackaberry 43-44). There are many different types of tropical storms that affect all different kinds of areas, but the ones with the most impact on the tropics are hurricanes. The tropical hurricane is the most well known of all cyclones. Each year approximately 80,000 cyclones are responsible for 20,000 deaths while causing massive damage to property and serious shipping hazards, due to high winds, high seas, and flooding from heavy rainfall and costal storms. The hurricane’s main energy source is latent heat that is resulting from condensed water vapor. Because of this they are generated and continue to gather their strength, only surrounded by the confines of warm oceans. A typical hurricane has a diameter of about 400 miles, a development of cumulonimbus clouds with their tops reaching 40,000 ft, and a pressure of approximately 950 mb. The main tropical cyclone activity in both hemispheres takes place in the late summer and autumn around times of maximum northward and southward displacements of the Equatorial Trough (Barry, Chorley 233). Two of the more severe hurricanes that have affected the Bahamas themselves were Hurricane Andrew and Hurricane Floyd. Andrew took place August 16-28th, 1992. On the 23rd and 24th of August, Andrew passed over the north end of Eleuthera and the southern Barry Islands in the Bahamas generating large amounts storm surge flooding. Records show two high water level marks taken after the storm; one was read at 16ft and the other at 23ft. Andrew was seen as a small but brutal Cape Verde hurricane that brought extraordinary economic destruction along a path through the northwest Bahamas, southern Florida, and south-central Louisiana. Below is a chart from the National Hurricane Center from Hurricane Andrew.

Graph!!

As shown in the chart, the Bahamas had three direct deaths and the amount of injuries is unknown. The damage in the Bahamas was approximately one billion dollars, which does not seem substantial when compared to the United States with damages of about 25 billion dollars, however, for a place of smaller size, it is devastating (Rappaport). Hurricane Andrew went through many different stages, starting off as a tropical depression, then moving upward to a tropical storm, then different levels of hurricane status, then back to a tropical storm, and finally back to a tropical depression. This chart goes through the latitude, longitude, time, wind, PR, and the state of the storm at those times.

Graph!!

Hurricane Floyd became a tropical depression on September 7th 1993 and then ended its tour on the 17th where it became an extratropical storm. The eye of Floyd moved right over Eleuthera, Abaco, Cat, and San Salvador in the Bahamas the day after it had reached its highest intensity of 155mph, a strong class four hurricane. By the time it reached the Bahamas it was presumed to be between a category three and four hurricane, which caused severe flooding, and damage. Through all of this there was only one death reported. Like the above chart on Andrew, below is partial tracking of Hurricane Floyd.

Graph!!

Hurricanes are amazing forces of nature, but the severe flooding and massive amounts of destruction and death caused by them is not so amazing or beautiful. This explains the use now of major tracking systems that can tell approximately where and when a hurricane will strike. Throughout the tropics there are many things that affect the weather and the environments located there. We have seen how the Coriolis effect, the Trade Winds, and the ITCZ all work together to create the atmospheric circulation and with this comes the large amounts of tropical rainfall. Over many of the past years we have seen how El Nino has devastated so many places and the major affects it has on the fishing industry. Then Hurricanes Andrew and Floyd swept through the Bahamas and caused widespread destruction. There are many components that help make up the weather and its behavior in the tropics.